2.1 Principle of a Seismic Marine Acquisition
To perform a seismic marine acquisition in a survey area, it is common to use seismic sources (guns, vibratory sources, . . . ) and seismic sensors. The sensors are housed in cables, called streamers (or linear antennas). Several streamers are used together to form an array of thousands of sensors. Sources are towed by one or several shooter vessels, and streamers are towed by one or several listener vessels. A same vessel can be both shooter vessel and listener vessel (i.e. can tow one or several streamers and one or several seismic sources).
To collect the geophysical data in the marine environment, the seismic sources (towed by at least one shooter vessel) are activated to generate single pulses or continuous sweep of energy. The signals generated by each source travels through the layers of the earth crust and the reflected signals are captured by the sensors (hydrophones) in the streamers (towed by at least one listener vessel).
After each shot, two files are created: a first file containing seismic data provided by seismic sensors (comprised in seismic streamers); and a second file, called RH file (for “record header”), containing information about the shot on shooter vessel (gun header (GH), real shot time (FTB, for “Field Time Break”) and source position (SP) at shot time). These first and second files are then combined to form a complete third file (also called SEG-D file). The interpretation of the seismic data contained in the SEG-D files is used to compute a 3D image of the earth crust.
Each theoretical location, where a seismic source must shoot, is a shot point location (also referred to as “shot point”), defined by its geographical coordinates (latitude/longitude and/or easting northing). When the source reaches this shot point, the gun is activated and produces an explosion. The set of shot points of all seismic sources is called “preplot”.
The acquisition process is controlled and monitored by a navigation system (also referred to as INS, for “Integrated Navigation System”), which is onboard a master vessel (also referred to as “master speed vessel”) and whose role is to compute position of sensors and sources, drive vessels along their acquisition path, according to the preplot geometry, and to activate sources to perform seismic acquisition at desired location (shot points).
The navigation system determines the moment of firing for each shot point, according to the positions of the various system components. This moment, also referred to as “shot time”, is often noted T0.
The actual positions of all equipments (hydrophones and guns) are known thanks to well-known measure means (GPS, RGPS, acoustics, compasses, depth sensors . . . ).
2.2 Multi-Vessel Operation
To further increase the quality of seismic imaging, the seismic surveys are now performed in multi-vessel operation, in order to obtain a wide azimuth illumination of the earth's crust (this explaining why, in this case, the preplot is referred to as “wide azimuth preplot” or “WAZ preplot”). A multi-vessel seismic system often comprises several shooter vessels and several listener vessels. A same vessel can be both shooter vessel and listener vessel.
The wide azimuth preplot defines a sequence of shot points, where the shots of the various vessels are interlaced.
The shooting order of the sources, and consequently of the vessels, is also defined in the wide azimuth preplot. The shooting order of the vessels must be respected and performed as close as possible to the geographic coordinates of the shot points specified in the wide azimuth preplot. So that the shooting order is complied, the various vessels must be synchronized.
In the simplified example of wide azimuth preplot of FIG. 4, there are three shooter vessels V1, V2 and V3, each towing a source S1, S2 and S3 respectively. We assume the shooter vessel V1 is the master speed vessel. We also assume that the rank of a shot is identical to the rank of the corresponding shot point (for example, the seventh shot is called “shot 7” and must be made at “shot point 7”). This example of wide azimuth preplot can be resumed as follows:                the shooter vessel V1 is in charge of shot 1, shot 4, shot 7, shot 10, shot 13, etc, which must be carried out respectively at shot point 1, shot point 4, shot point 7, shot point 10, shot point 13, etc;        the shooter vessel V2 is in charge of shot 2, shot 5, shot 8, shot 11, etc, which must be carried out respectively at shot point 2, shot point 5, shot point 8, shot point 11, etc; and        the shooter vessel V3 is in charge of shot 3, shot 6, shot 9, shot 12, etc, which must be carried out respectively at shot point 3, shot point 6, shot point 9, shot point 12, etc.        
In this example, the three shooter vessels V1, V2 and V3 are supposed to be aligned, but the three shot points corresponding to three successive shots (each carried out by a different one the three shooter vessels) are not aligned (e.g. shot points 1, 2 and 3 are not aligned). However, and as shown in FIG. 4, we assume shooter vessel V2 is ahead and shooter vessel V3 is late.
We present now the calculation of a theoretical shot time associated with a shot point.
For each seismic source, a path (also referred to as “sail line” or “navigation line”) is defined, which passes through <<waypoints>>, including the shot points associated with this seismic source.
The projection, on this path, of the speed of a given point X (e.g. a reference point of the seismic source) is called “speed along” (and noted SA(X)).
The distance between two points X and Y projected on this path is called “distance along” (and noted DA(X,Y)).
The point used to compute the theoretical shot time (T0) is called “predict point” (and noted PP). It can be a reference point located on the seismic source or on the shooter vessel which tows this seismic source.
For a given seismic source S and a given predict point PP, the theoretical shot time T0 associated with a given shot point SP is computed according to the following formula:T0=(DA(SP,PP)/SA(PP))+current time
For example, in FIG. 4, for the seismic source S1 and a given predict point PP corresponding to a reference point of the seismic source S1, the theoretical shot time T0 associated with the “shot 7” is computed according to the following formula (also referred to as “calculation in distance mode”):T0(shot 7)=(DA(“shot point 7”,PP)/SA(PP))+current time
It must be noted that DA(“shot point 7”, PP) is noted DA1 in FIG. 4.
2.3 Definitions
                Bull's Eye (noted BE): a master vessel is a reference for other vessels (slave vessels). A point of the master vessel (or of any equipment associated with the master vessel, e.g. a source) is used as reference point to calculate the ideal position of other vessels (slave vessels), i.e. for space synchronization of the slave vessels.        
The ideal position of a slave vessel is indicated by a circular target called “bull's eye” (BE), having:                a center which depends on the projection of the master vessel's reference point on the sail line of the slave vessel. In the particular case where the slave shooter vessels are supposed to be aligned with the master vessel (see FIG. 4), the center of the “bull's eye” is coincident with the projection of the master vessel's reference point on the sail line of the slave vessel. In the particular case where the shooter vessels are not supposed to be aligned (see FIGS. 1, 2A-2D and 3A-3C), there is a predetermined offset, along slaves vessels sail line, between the center of the “bull's eye” and the projection of the master vessel's reference point on the sail line of the slave vessel. For example, in FIG. 1, this offset is equal to 18.75 m for the “bull's eye” of the slave shooter vessel V2, 37.5 m for the “bull's eye” of the slave shooter vessel V3 and 56.25 m for the “bull's eye” of the slave shooter vessel V4; and        a radius of tolerance which can be determined by contract requirements (e.g. 10 m).        
A reference point of the slave vessel, defined in advance, must be located in the “bull's eye” to ensure proper synchronization of the slave vessel. In the example of FIG. 4, the shooter vessel V1 is the master vessel. The ideal position of the slave shooter vessel V2 is indicated by a circular target (“bull's eye”) noted BE2. The ideal position of the slave shooter vessel V3 is indicated by a circular target (“bull's eye”) noted BE3.                Bull's Eye Distance Along (noted BE DA): for a slave shooter vessel, it is the distance between the center of the bull's eye and the reference point of this slave shooter vessel, projected on the path (sail line) of this slave shooter vessel. In the example of FIG. 4, the “Bull's Eye Distance Along” of the slave shooter vessel V2 is noted BE DA2. The “Bull's Eye Distance Along” of the slave shooter vessel V3 is noted BE DA3. The aforesaid condition that “the reference point of the slave vessel must be located in the “bull's eye” to ensure proper synchronization of the slave vessel”, can also be expressed as “the Bull's Eye Distance Along (BE DA) should be less than or equal to the radius of tolerance of the “bull's eye” (BE)”.        Theoretical shot spacing: it is the theoretical distance along between two consecutive shot points. Basically, each real distance along between two shot points should be close to it. In other words, the theoretical shot spacing is the distance between two theoretical shots along the line of the master. For example if master vessel has to shot the shot point N and the shot point N+4, and slave vessels have to shot the points N+1, N+2 and N+3, we have: theoretical shot spacing=distance(shot point N, shot point N+4)/(4−0).        Shot time interval (noted STI): it is the real time interval between two shots.        Minimum Shot Time Interval (noted “Min STI”, and also called “Minimum shot cycle time”): it is the minimum time interval that must be maintained between two successive shots to avoid any interference. If this value is not respected, there is a shot overlap and the two shots are not considered valid.2.4 Known Methods for Shot Overlap Avoidance            2.4.1 FIG. 1 shows an ideal scenario in a particular context defined as follows: there are four shooter vessels V1, V2, V3 and V4, each towing a source S1, S2, S3 and S4 respectively. We assume the shooter vessel V1 is the master speed vessel. We also assume that the rank of a shot is identical to the rank of the corresponding shot point (for example, the fourth shot is called “shot 4” and must be made at “shot point 4”). We also assume a theoretical shot spacing equal to 18.75 m.
For simplicity, only the first four shot points (shot point 1 to shot point 4) are illustrated:                the shooter vessel V1 is in charge of shot 1, to be carried out at shot point 1. The realization of this first shot is symbolized by the term “bang” in FIG. 1;        the shooter vessel V2 is in charge of shot 2, to be carried out at shot point 2;        the shooter vessel V3 is in charge of shot 3, to be carried out at shot point 3;        the shooter vessel V4 is in charge of shot 4, to be carried out at shot point 4.        
In this example, the four shot points (shot 1 to shot 4) are supposed to be aligned, but the four seismic sources S1 to S4 (and therefore the four shooter vessels V1 to V4) are not aligned.
If we achieve a situation where each slave shooter vessel remains at a constant speed in relation to the master shooter vessel (keeping the inline distance between the sources constant) and where there are no communication outages between the slave shooter vessels and the master shooter vessel, then there is no problem (shots 2, 3 and 4 are actually carried out at shot points 2, 3 and 4 respectively). However, this is unrealistic.    2.4.2 Referring now to FIGS. 2A to 2D, we present a first known method for managing shots in a multi-vessel seismic system comprising several shooter vessels (a scheduler shooter vessel and at least one slave shooter vessel) and at least one listener vessel.
This first known method allows to minimize the “Distance Along” (DA) error on shot locations (i.e., for a given shot by a slave shooter vessel, the distance between the theoretical shot point and the location where the shot was actually made, projected on the sail line of the slave vessel).
In this first known method, the navigation system (INS) fires the sources based on the along line progress of the slave shooter vessels V2, V3 and V4.
As shown in FIG. 2A, we assume that the master shooter vessel V1 shoots normally when it is on the shot point 1, the slave shooter vessel V2 is late and falls 30 m behind its “bull's eye” (48.75 m between the source S2 and the shot point 2, instead of 18.75 m in FIG. 1), and the slave shooter vessel V3 is late and falls 10 m behind its “bull's eye” (47.5 m between the source S3 and the shot point 3, instead of 37.5 m in FIG. 1).
As shown in FIG. 2B, when the master shooter vessel V1 has moved forward 47.5 m, the slave shooter vessel V3 (and more precisely its source S3) reaches the shot point 3 before the slave shooter vessel V2 (and more precisely its source S2) reaches the shot point 2. In theory, the slave shooter vessel V3 should fire its source S3 (since the source S3 is located at the shot point 3), but in reality, the navigation system (INS) of the master shooter vessel V1 assumes the slave shooter vessel V2 is next to fire (1.25 m from shot point 2), and decides there is no shot for the slave shooter vessel V3, at the shot point 3.
As shown in FIG. 2C, when the master shooter vessel V1 has moved forward 48.75 m, the slave shooter vessel V2 (and more precisely its source S2) reaches the shot point 2. The slave shooter vessel V2 then eventually fires, but over a full shot cycle later than planned. It must be noted that the slave shooter vessel V4 will reach its shot point 4 in 7.5 m, i.e. around 4 seconds.
As shown in FIG. 2D, when the master shooter vessel V1 has moved forward 56.25 m, the slave shooter vessel V4 (and more precisely its source S4) reaches the shot point 4. The navigation system (INS) of the master shooter vessel V1 can be configured either to fire the shot for the slave shooter vessel V4, or to inhibit the shot for the slave shooter vessel V4 (considering it is too close (4 seconds) from the record for the shot of the slave shooter vessel V2).
The entire issue of FIGS. 2A to 2D will repeat whilst the slave shooter vessel V2 stays out of position.
These FIGS. 2A to 2D demonstrate the complexity of the issue when trying to fire each source exactly on its preplot targets (i.e. exactly on the shot points associated to this source).
As detailed above, the drawbacks of this first known method are:                missed shots due to slave shooter vessel being out of position (e.g. in FIG. 2B, no shot for the slave shooter vessel V3 due to slave shooter vessel V2 out of position); and        shot overlap due to shots fired in the middle of the important part of a record from a previous shot (e.g. in FIG. 2D, the shot of the slave shooter vessel V4 is close to the record relating to the shot of the slave shooter vessel V2).        
In an alternative embodiment of this first known method, the navigation system (INS) also checks whether the condition “BE DA≦BE radius” is satisfied. This ensures a suitable shot time interval (STI), while guaranteeing the firing order if the BE radius is adapted to the speed of the sources (i.e. if the BE radius is not too long compared to the speed of the vessels). With this alternative embodiment, the source S2 (of slave shooter vessel V2) would not have done its shot (because: BE DA2=30 m>BE radius=10 m) and the source S3 (of slave shooter vessel V3) would have done its shot (because: BE DA3=10 m<=BE radius=10 m).    2.4.3 Referring now to FIGS. 3A to 3C, we present a second known method for managing shots in a multi-vessel seismic system comprising several shooter vessels (a scheduler shooter vessel and at least one slave shooter vessel) and at least one listener vessel.
This second known method allows to fire the sources as a function of the along line progress of the master shooter vessel. The shots of each shot point are done whatever the location of the shooter and without taking account of the DA error. In other words, this second known method proposes to change the operation mode of the navigation system (INS) of the master shooter vessel V1. More precisely, the navigation system (INS) fires the sources based only on the along line progress of the master shooter vessel V1, without checking whether the condition “BE DA≦BE radius” is satisfied. The real shot times of the slave shooter vessels V2, V3 and V4 are set to theoretical shot times predicted by the navigation system (INS) of the master shooter vessel V1 (regardless of the actual position of slave shooter vessels).
We assume the same scenario as in FIG. 2A: the master shooter vessel V1 shoots normally when it is on the shot point 1, the slave shooter vessel V2 is late (30 m behind its “bull's eye”) and the slave shooter vessel V3 is late also (10 m behind its “bull's eye”).
As shown in FIG. 3A, when the master shooter vessel V1 has moved forward 18.75 m, the slave shooter vessel V2 has its source S2 fired, even though the slave shooter vessel V3 is ahead of the slave shooter vessel V2. This means that the slave shooter vessel V2 fires “out of position”, by 30 m (from the shot point 2) in this case, but the operational concerns of first known method disappear (no missing shots).
As shown in FIG. 3B, when the master shooter vessel V1 has moved forward another 18.75 m, the slave shooter vessel V3 has its source S3 fired unlike in the first scenario of FIGS. 2A to 2D), being “out of position” only by 10 m (from the shot point 3).
As shown in FIG. 3C, when the master shooter vessel V1 has moved forward another 18.75 m, the slave shooter vessel V4 has its source S4 fired.
A drawback of this second known method is that the slave shooter vessels can miss their shot points, i.e. their inline targets (e.g. in FIG. 3A the slave shooter vessel V2 fires “out of position” by 30 m, and in FIG. 3B the slave shooter vessel V3 fires “out of position” by 10 m).    2.4.4 Other drawbacks common to the first and second known methods
In aforesaid first and second known methods, the shooting management is centralized in the navigation system (INS) of the master shooter vessel. The latter makes all the decisions and sends activation signals (shoot commands) to the slave shooter vessels, in order to command the firing of the various shots carried out by the slave shooter vessels. In practice, these activation signals are comprised in a flow of information which is exchanged in real time via a radio link (wireless channels) between the master shooter vessel and the slave shooter vessels.
A drawback of the radio link is that it is not 100% reliable due to fading, long distances between vessels, multipath and floating obstructions, etc. In other words, during a seismic survey, the radio link between the vessels can be lost or down (broken). If this happens when a shoot command is transmitted to the slave vessel's source, the shoot command will not be received, the shot will not be made, and the vessels will miss a spot (shot point) where data are required.
Another drawback of the radio link is that it requires regular calibration. Calibration is normally carried out “off-line,” the result of which is that timing errors may occur between calibrations, these timing errors being undetected.